I. Field of the Invention
The present invention relates generally to remotely movable and articulatable robots used for cleaning surfaces with a sprayable liquid, and more particularly to such devices as used for cleaning the interiors of large oil and hazardous material storage tanks. This invention also relates to methods used for cleaning such tanks and to the recycling of cleaning liquids and waste materials accumulated during the cleaning process.
II. Description of Prior Art
Large cylindrical tanks are the primary containers used by refineries for the storage of various grades of crude oil prior to refinement into fuels and other lighter petroleum products. Over time, an accumulation of sediment is commonly experienced within the tank, as well as a congealing of the oil into a tar-like sludge. In order to maximize use of the tank and to minimize the impurities in the crude oil to be refined, cleaning of the tanks is occasionally required, wherein the sludge and sediment are removed by spraying the tank interior surfaces with a highly pressurized liquid. The cleaning liquid most often used is a petroleum-based solvent capable of dissolving the sludge, and is sometimes referred to as "cutter stock". Less often, water is used to achieve a similar, but less effective, result.
Typically, a number of workers will use high pressure hoses inside the tank and spray the cutter stock on the interior surfaces to dislodge the sludge material from the tank, and a pump is placed in a lower sink area or sump in order to suction out all of the mixture of cutter stock and sludge. This process is clearly strenuous and burdensome to the workers, primarily because of the toxic hydrocarbon vapors which are always present inside the tank, the weight of the spray hoses, and the high temperatures generated within the tank interior during the warmer months. Protective garments and masks are commonly worn to protect the workers' skin from exposure to harmful chemicals, but this only adds to the discomfort caused by the heat. Consequently, workers may only stay within the tank for short periods before leaving to rest, during which times replacement workers step in to continue the job. As a whole, this manual method of cleaning the tanks is uncomfortable, strenuous and dangerous for the reasons discussed.
In an attempt to alleviate most of the problems associated with the manual method, a device patented by Krajicek, et al., in U.S. Pat. No. 4,817,653 proposed a mobile tank cleaning, water washing robot having an articulatable spray nozzle. This device includes an open bottom frame having tank-type treads for locomotion, and an attached pump for removing accumulated waste water and material out of the tank. The motors which operate the treads, the attached waste removal pump and the spray nozzle are all powered by a hydraulic power system located outside the tank. The wash water supply is also situated outside of the tank, but the robot is controlled by an operator standing inside the tank. Some of the advantages disclosed by the Krajicek patent are that significantly higher water pressures and flow rates may be used because the spraying means is not held by human hands, only one operator need be stationed inside the tank in order to control the robot, and that the operator may safely stay within the tank for much longer periods due to the relative absence of any heavy labor compared to the manual method.
Several deficiencies exist, however, in the Krajicek device which must be overcome in order to provide the most efficient method of cleaning the interiors of hydrocarbon storage tanks, as well as to ensure the safety of the operator. Specifically, it is unclear whether Krajicek may be adapted for use with the cutter stock liquid described above. The use of water in that device tends to produce static electricity, which is extremely hazardous in the flammable environment inside the storage tank. Also, the waste removal pump in Krajicek is limited to removing waste materials directly in the path of the robot, because it is permanently attached to the frame. It is often desireable to suction the waste material from areas containing the greatest concentration of waste material and expended solvent, and the prior device must be diverted from continuous spraying in order to remove waste material from those areas. Additionally, while the Krajicek device is generally not hindered by debris and accumulated sludge in its path due to its large size and use of tank treads for locomotion, such a device is very bulky and heavy to move and potentially expensive to repair. Disassembly of that device is required before and after the tank cleaning process, and a scaffolding system must be constructed in order to move pieces of the device into and out of the tank.
Furthermore, regardless of which prior equipment or methods have been used to clean these tanks, no acceptable solution has been provided for the problem of disposal of the accumulated waste mixture. In too many instances, this waste mixture is simply dumped into a waste pit at the refinery site, or collected and disposed of through hazardous waste facilities. Given the heightened awareness of environmental issues by members of the public, government and industry, every attempt must now be made to minimize the effect of tank cleaning on the environment by incorporating in-process recycling of the cleaning liquids.